1. Field of the Invention
This invention relates to thermal printing and more particularly to a system and method for automatically detecting defective thermal printhead elements in a thermal line printer or in a serial thermal printhead and for automatically correcting for at least one defective thermal printhead element.
2. Description of the Prior Art
In conventional thermal line printers the character positions are fixed in relation to the line of thermal resistive elements along the thermal printhead. To illustrate, assume that in a conventional thermal line printer each character position is 14 dots wide with an unused gap of 5 dots between acjacent character positions. Thus, in this example of a conventional thermal line printer, character 1 will always use resistive elements 1-14, character 2 will always use resistive elements 20-33, character 3 will always use resistive elements 39-52 , and so forth.
The problem with a thermal line printer having fixed character positions is that, when a resistive element burns out, there is no way that the conventional thermal line printer can automatically detect the failure of that resistive element. The failure of a resistive element only becomes known after an operator of the printer notices "holes" or gaps appearing in some of the printed characters. The operator might not notice these "holes" until after, for example, thousands of print lines have been printed with "holes" in some of the printed characters. After finally noticing such "holes", the operator then has to shut off the thermal line printer and summon a skilled technician to replace the defective thermal printhead. The down time of the thermal line printer may be quite long and, therefore, costly in terms of repair expenses and lost man hours of the operator.
The background art known to the applicants at the time of the filing of this application is as follows:
U.S. Pat. No. 4,284,876, Thermal Printing System, by N. Ishibashi et al;
U.S. Pat. No. 4,321,610, Dot Matrix Printer with Half Space Dot Capability, by D. F. Moore et al; and
U.S. Pat. No. 4,364,063, Thermal Recording Apparatus, by G. Anno et al.